MIDW employs Pt/ zeolite beta at low pressure (about 400 psi), preferably in the absence of sulfur contamination, although trace amounts of sulfur may be permitted, to drive dehydrogenation and isomerization reactions. The products are low pour point kerosene, diesel, and fuel oil products. A hydrogen partial pressure greater than 400 psi in an MIDW bed may actually reduce the dehydrogenation function of MIDW. For this reason, the preferred feed to commercial MIDW installations is the hydrotreated effluent of the Moderate Pressure Hydrocracking Process (MPHC). This is a clean feed, having a low heteroatom content. The instant invention provides a means for sour feed to be introduced directly into the MIDW process.
Many opportunities exist in refineries not employing MIDW to produce low pour point products in order to reduce the amount of valuable cutter stock required to satisfy the fuel oil pour point specifications. These refineries have fuel oils with high sulfur content, which render the MIDW process inoperable, due to the poisoning of the Pt metal which is required for the dehydrogenation reactions of MIDW by the sulfur compounds or H.sub.2 S. (The partial denitrogenation reactions for the fuel oil in the HDS beds also produce NH.sub.3 byproducts, which reduce the acid isomerization function of the zeolite, if the HDS effluent is cascaded to the MIDW bed without proper stripping of NH.sub.3.)
There have been previous examples of counter-current mode operation in a commercial hydroprocessing unit. Criterion/Lummus disclosed the SynSat process in the Oil and Gas Journal, Jul. 1,1991, p.55. This process involves the integration of hydrodesulfurization and aromatic saturation. The aromatic saturation bed requires a higher purity make-up gas in the bottom of the aromatic saturation bed (than is required in the MIDW bed of the instant invention) because of the need for high hydrogen consumption in the aromatic saturation step.
U.S. Pat. No. 4,764,266 and U.S. Pat. No. 4,851,109 disclose the MIDW process as it is often practiced. Feed is subjected to the Moderate Pressure Hydrocracking step and some materials in the kerosene and distillate boiling ranges are removed before the unconverted paraffinic residue of MPHC effluent is subjected to MIDW. MPHC effluent is a clean feed of low heteroatom content. A hydrofinishing step follows MIDW. There is no teaching of the use of recycle gas in the MIDW step, or of the use of catalytic hydrodesulfurization (CHD) prior to MIDW. U.S. Pat. No. 4,851,109, which is based on a continuation application of U.S. Pat. No. 4,764,266, contains an additional solvent extraction step following MIDW before the lube is subjected to hydrofinishing.
Although MPHC effluent (which is a clean feed) is the preferred source of feed to the MIDW process, fuel oils produced by methods other than MPHC are at times available for use. These may have a high heteroatom content. Consequently, stripping of H.sub.2 S and NH.sub.3 is required at two separate points, following the hydrocracking stage, and following the MIDW reaction section.
A significant difference in the process parameters exists for HDF(HDS or CHD) and MIDW, which impacts on the capital investment. Consequently, the equipment requirements include separate heating and cooling for the feed and the effluent, separate high pressure separators, and separate recycle compressors to accommodate the requirements for the two stages. Basic design parameters for the two reaction sections are described as follows:
TABLE 1 ______________________________________ HDS MIDW ______________________________________ Temperature, F. 650-750 600-750 Hydrogen partial pressure, psi 600-800 400 Hydrogen consumption, SCF/B 300 200 Hydrogen circulation, SCF/B 1200 2000 Impact of H.sub.2 S on inhibition Minor Significant of catalyst ______________________________________